Measuring element for anion-sensitive solid-contact electrode and ion-sensitive solid-contact electrode

ABSTRACT

A measuring element is disclosed for an ion-sensitive solid-contact electrode for measuring ion activity in a measurement medium. An ion-sensitive solid-contact electrode having such a measuring element and an electrochemical sensor having such a solid-contact electrode are also disclosed. The measuring element can include an ion-sensitive layer arranged to contact a measurement medium when in operation, and conductive to lithium ions; and a single-phase electrically conductive layer, which includes metallic lithium or a lithium-(0)-alloy. A solid-state electrolyte layer can be arranged between the ion-sensitive layer and the electrically conductive layer.

RELATED APPLICATION

This application claims priority as a U.S. continuation patentapplication of U.S. patent application Ser. No. 16/381,145, filed Apr.11, 2019, which is a continuation application under 35 U.S.C. § 120 toPCT/EP2017/076165, which was filed as an International Application onOct. 13, 2017 designating the U.S., and which claims priority toEuropean Application 16193653.9 filed in Europe on Oct. 13, 2016. Theentire contents of these applications are hereby incorporated byreference in their entireties.

FIELD

A measuring element is disclosed for an ion-sensitive solid-contactelectrode for measuring ion activities, as well as an ion-sensitivesolid-contact electrode having a measuring element, such as a pHsolid-contact electrode.

BACKGROUND INFORMATION

Glass electrodes having a thin ion-sensitive glass membrane have beenused in the laboratory and in process plants to measure ion activitiessuch as pH. As a thin-walled closure, the ion-sensitive glass membranecan be fused to an electrode shaft made of insulating glass, wherein theexpansion coefficients used for the electrode shaft and the glassmembrane are similar.

Such ion-sensitive glass electrodes may be used as the measurementelectrode together with a reference electrode that may be used as anelectrochemical sensor, wherein the measurement electrode and thereference electrode may be configured as separate elements or as aso-called combination electrode accommodated in a shared housing.

During operation and/or during a measurement, the outer surface of theglass membrane comes in contact with the measurement medium or themeasurement solution, and the inner surface is in contact with anelectrolyte solution as the reference solution. By replacing alkali ionsof the glass with hydrogen ions (H+), thin gelatinous silicate swellinglayers are formed on the outside of the glass membrane. Due todifferences in chemical potentials of H+ ions between the swelling layerand the adjacent solution, a Galvani voltage develops on each side ofthe glass membrane due to passage of H+ ions through thesolution/swelling layer phase boundary. The total of these galvanizationvoltages yields the glass electrode voltage that can be measured betweenthe measurement medium and the electrolyte, with the two referenceelectrodes as the inner lead and the outer lead.

Ion-sensitive glass electrodes as well as sensors having ion-sensitiveglass electrodes can have good measurement properties. For example,glass electrodes used to measure pH can have reliable slope, long-termstability, selectivity and limit of detection. However, known glasselectrodes can only be used in a predetermined orientation. Furthermore,glass electrodes have mechanical issues, such as being fragile andeasily breakable, as a result of which glass splinters can, for example,contaminate a measurement medium.

Known attempts to manufacture ion-sensitive solid-contact electrodes orion-sensitive solid-state electrodes have not been successful becausewhen such electrodes are used to measure ion activity, their performancehas been inferior and/or they can be used only with certain restrictionssuch as, for example, with respect to covering the pH measurement rangeor the pressure and/or temperature stability.

DE 196 20 568 A1 discloses a pH glass electrode with a two-layer glassmembrane, having an interior coated with silver and being filled with anelastic material to address mechanical stability of the electrode. Thetwo-layer glass membrane includes (e.g., consists of) an ion-conductiveglass layer in contact with a medium and an electrically conductive andion-conductive glass layer on the interior.

WO 1/04615 A1 discloses an ion-sensitive solid-state electrode having aglass membrane and a metallic core, which is also sealed with a stoppermade of an electrically conductive metal. For example, one electrodedescribed has a lithium-containing glass membrane, a lithium-led alloyas the metallic core, and a stopper made of a so-called Wood alloy.

U.S. Pat. No. 4,632,732 A discloses an ion-sensitive pH electrode havingan intermediate layer between the ion-sensitive glass membrane and theelectric contact, the intermediate layer being contacted by electricallyconductive silver epoxy. The intermediate layer includes (e.g., consistsof) lithium-vanadium oxide, which is permanently bonded to the glassmembrane by sintering.

DE 197 14 474 C2 discloses an electrochemical sensor produced by athick-film technique. A metal electrode of gold, platinum, a gold-silveralloy or a platinum-silver alloy is deposited on a steel ceramicsubstrate by a screen-printing technology, and then at least one glasscomposition is applied as the ion-sensitive glass membrane.

DE 100 18 750 C2 discloses a solid-contacted ion-sensitive glasselectrode, in which a polymer contact layer having an intrinsicallyconductive organic polymer that adheres both to the glass membrane andto the tap line is applied to the glass membrane's interior.

U.S. Pat. No. 4,133,735 A discloses a planar pH electrode. A conductivelayer in thin-film or thick-film technology is applied to a forsteritewafer and then a layer of pH glass in thick-film technology is appliedto at least a portion of this conductive layer. The conductive layerincludes (e.g., consists of) chromium, nickel, gold, silver or agold-platinum mixture. In addition, a lead or a tap line or an FET(field effect transistor) can be bonded to the conductive layer. Inorder to preclude the conductive layer from contacting the measurementmedium, the electrode is sealed with respect to the measurement medium,except for the glass membrane.

U.S. Pat. No. 4,280,889 A discloses a multilayer electrode for measuringthe ion concentration, whereupon an electrically conductive structure ofsuccessive layers of chromium and silver is applied to an insulatingsubstrate, such as ceramic or glass. A solid electrolyte layer of silverchloride and an outer ion-sensitive glass layer, such as a pH glass, isapplied over the conductive structure. At least the glass layer isapplied by RF sputtering.

EP 0 420 983 A1 discloses an ion-sensitive solid-phase electrode havingan ion-sensitive glass membrane, whose glass composition containslanthanum and neodymium. A solid phase contact is applied to the glassmembrane, such as an oxide compound of the formula MxLayNdzWO3, where Mis an alkali metal or alkaline earth metal and 0<x+y+z<1. Thesolid-phase contact is in turn connected to a lead wire.

DE 37 27 485 A1 discloses a solid-state conductive system for use withan ion-sensitive element for electrochemical sensors, wherein amultiphase solid contact material is provided between an electricconductor element and the ion-sensitive element. The conductive systemincludes an ion-conducting intermediate layer (e.g., CsHSO4), whichreduces the internal resistance of the system. A multiphase systemincluding (e.g., consisting of) lithium (Li) and tin (Sn), having a tinphase with lithium dissolved in it and an intermetallic phase (e.g.,LixSny), is used as contact material. Because of the solubility ofLixSny in Sn, the activity of lithium in tin and the potential of thetap line system should be stabilized. However, the lithium solubility isa function of temperature, so the thermodynamic equilibrium changes as afunction of temperature. When used in an electrochemical sensor, thisresults in a prolonged rise time or drift when there are changes intemperature.

An enameled pH electrode distributed by Pfaudler, Germany represents oneof the few technologies used commercially for an ion-sensitivesolid-state electrode. However, this cannot be used over an entire pHrange, and furthermore, it is expensive in comparison with traditionalglass electrodes.

Attempts have not yet been successful to develop an ion-sensitive solidcontact electrode for measuring ion activity, with comparable or bettermeasurement properties than the known ion-sensitive glass electrodes.For example, it has not yet been possible to achieve a conductivetransition from a measurement solution to the glass membrane and asolid-phase electrode to a tap wire in a satisfactory and long-lastingmanner.

SUMMARY

A measuring element is disclosed for an ion-sensitive solid-contactelectrode for measuring ion activity in a measurement medium, themeasuring element comprising: an ion-sensitive layer arranged to contacta measurement medium when in operation, and conductive for lithium ions;and a single-phase electrically conductive layer, which includesmetallic lithium or a lithium-(0)-alloy, wherein the measuring elementis a solid-state measuring element which includes a solid-stateelectrolyte layer arranged between the ion-sensitive layer and theelectrically conductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described in greater detail below with respectto the figures, wherein the same elements are labeled with the same orsimilar reference numerals, and wherein:

FIG. 1 shows a schematic view of an exemplary electrochemical sensorwith an ion-sensitive solid contact electrode having an exemplarymeasuring element as disclosed herein;

FIG. 2 shows a schematic view of an exemplary ion-sensitive layerstructure in a sectional view;

FIG. 3 shows a schematic view of an exemplary measuring element asdisclosed herein, and having an ion-sensitive layer structure accordingto FIG. 2 shown in cross section;

FIG. 4 shows a schematic view of another exemplary measuring element asdisclosed herein, and having an ion-sensitive layer structure accordingto FIG. 2 shown in cross section;

FIG. 5 shows a schematic view of exemplary two components of themeasuring element according to FIG. 4;

FIG. 6 shows a schematic view of another exemplary measuring element asdisclosed herein, and having an ion-sensitive layer structure accordingto FIG. 2 shown in cross section;

FIG. 7A shows exemplary measurement signals of a known glass sensor withrespect to an Ag/AgCl reference electrode at 25° C.; and

FIG. 7B shows exemplary measurement signals of a sensor as disclosedherein with respect to an Ag/AgCl reference electrode at 25° C.

DETAILED DESCRIPTION

For process plants, exemplary embodiments can provide ion-sensitivesolid contact electrodes for measuring ionic activities, wherein suchelectrodes would be mechanically stabile and suitable for use regardlessof position, while having measurement characteristics at leastcomparable to those of known glass electrodes. In addition, use of anion-sensitive solid-contact electrode in a process plant or a processenvironment involves a high thermal stability of the electrode used aswell as of a measuring element contained therein in order to minimizeinfluences on the components and/or a resulting measurement signal dueto fluctuations in temperature in the process medium and/or the processenvironment.

A measuring element for an ion-sensitive solid contact electrode isdisclosed for measuring ion activity in a measurement medium, theelectrode having an ion-sensitive layer, which is in contact with themeasurement medium during operation and is conductive for lithium ions,and includes a single-phase electrically conductive layer havingmetallic lithium or a lithium-(0)-alloy. A measuring element asdisclosed herein can be an element, which also includes a solid-stateelectrolyte layer arranged between the ion-sensitive layer and theelectrically conductive layer.

A measuring element as disclosed herein is, for example, a solid-stateelement, which is robust and free of hysteresis as well as beingelectrochemically reversible, such as when used in a solid contactelectrode for measuring the ion activity in a measurement medium. Anexemplary measuring element as disclosed can have thermodynamicstability over a wide temperature range and have little delay time withtemperature changes or at least no prolonged delay time.

The ion-sensitive layer includes, for example, an ion-sensitive glass,which is conductive for lithium ions. Such glasses are used as the glassmembrane in known ion-sensitive glass electrodes, for example, such aspH electrodes.

An electrically conductive layer having pure lithium or alithium-(0)-alloy is for example a single-phase solid solution, asolid-state compound or pure substance and has a high lithiumconcentration or activity. Pure lithium is also referred to herein asmetallic lithium. A high lithium activity can ensure that the lithiumconcentration can be regarded as constant over the lifetime or useperiod of the measuring element, and thus the measuring element willhave a uniform measurement performance. The electrically conductivelayer can, for example, advantageously also be homogenous. A measuringelement as disclosed herein can include highly active lithium operatesclose to the standard reduction potential of lithium(Li(solid)→Li+=−3.04 V).

A single-phase lithium-(0)-alloys as disclosed can, for example, incomparison with pure lithium be more stable mechanically and physically,which can facilitate processing and also greatly improve the mechanicalstability of the resulting measuring element. Examples of suitablelithium-(0)-alloys include single-phase lithium-magnesium alloys orlithium-copper alloys. Because of their higher lithium concentration,single-phase lithium-rich lithium-magnesium alloys with a cubicbody-centered crystal structure can be particularly suitable as theelectrically conductive layer.

The solid-state electrolyte layer can include one of the followingexemplary Li ion-conducting crystalline or amorphous (vitreous)solid-state compounds or mixtures thereof: lithium borates, including(i.e., such as) B2O3-Li2O, lithium borate-sulfates (LiBSO), such asB2O3-0.8Li2O-0.8Li2SO4, lithium borophosphates (LiOP), such as½×Li2O—BPO4, Li—O-2O3-P2O5 in general, lithium aluminates, such asLi5AlO4, lithium borosilicates, such as Li2O—B2O3-SiO2, lithium galliumoxides, such as Li5GaO4, lithium germanates, such as Li(4-3x)AlxGeO4,lithium nitrides, lithium phosphates, such as Li(1+x)Ti2SixP(3-x)012 orLi(1+x)MxTi(2-x)(PO4)3, where M=Al, Ga, Fe, Sc, In, Lu, Y, La,lithium-phosphorus oxynitrides, such as Li3PO4-xNx, lithium-silicatealuminates, such as LiAlSi2O6, LiAlSiO4, Li9SiAlO8, lithium silicates,such as Li2SiO4, Li4SiO4, Li2SiO3, Li2Si2O5, lithium-silicon phosphates,such as Li3.6Si0.6P0.4O4, lithium-silicon-phosphorus oxynitrides, suchas Li4SiO4-x-Li3P4-yNx+y, lithium thiogermanates, Li2GeS3, Li4GeS4,Li6GeS5, lithium titanates, such as Li2Zr(1-x)TixO3, lithium vanadates,lithium compounds, such as Li2S-SiS2-Li3PO4 or Li7P3S11, lithium-zincoxides, such as Li6ZnO4, nitrated lithium borates (Li—B—O—N), such asLi3.09BO2.53N0.52, sulfides, such as Li2S—GeS2, orthio-lithium-germanium compounds, such as Li10GeP2S12,Li3.25Ge0.25P0.75S4.

Exemplary materials suitable for the solid-state electrolyte layer arecharacterized, those that contain movable lithium ions, are stable withrespect to elemental lithium and are nonconductive electrically. Thematerial of the solid-state electrolyte layer can for example form avitreous matrix and can be deposited by sputtering or comparablemethods. Such materials include the aforementioned phosphate, phosphide,oxide, borate, nitride, sulfide and sulfate compounds as well asmixtures thereof containing lithium at least proportionally as acationic component. These compounds can be modified further to improvethe conductivity, for example, by incorporating nitrogen.

The material or the solid-state compound used for the solid-stateelectrolyte layer is both conductive for lithium ions (Li+) andelectrically insulating. Furthermore, the material of the solid-stateelectrolyte layer can, for example, be as stable as possible withrespect to the material of the electrically conductive layer in order toprevent or at least greatly diminish its degradation or destruction. Inaddition, the solid-state electrolyte layer can serve to delineate theion-sensitive layer from the electrically conductive layer and therebyprevent destruction or decomposition of the ion-sensitive layer becauseof the high lithium activity of the electrically conductive layer. Inthis way, almost all competing interfacial equilibria at the interfacesbetween the ion-sensitive layer and the electrically conductive layerand/or the interfaces with the solid-state electrolyte layer can beruled out, which results in a high potential stability and very goodreproducibility of the measuring element.

In an exemplary embodiment, the solid-state electrolyte layer includes alithium-phosphorus oxynitride compound (LiPON), such as Li3PO4-xNx. Thiscan have an advantage that, in addition to its electrochemicalproperties, it has suitable conductivity properties and can be appliedwell to the ion-sensitive layer. LiPON is electrically insulating andionically conductive. Furthermore, a solid-state electrolyte layerdesigned and configured in this way can prevent the material of theelectrically conductive layer from attacking the material of theion-sensitive layer because of its high lithium activity, such as whenusing pure lithium. The interface between the electrically conductivelayer and the solid-state electrolyte layer can, for example, have ahigh exchange current density.

The measuring element can also include a protective layer, which isapplied to the electrically conductive layer as protection againstambient influences. The protective layer can serve to prevent a reactionbetween oxygen or humidity from the environment, for example, and theelectrically conductive layer. Such a protective layer can beadvantageous when, for example, pure lithium or a reactive lithium-basedcompound is used as the electrically conductive layer.

In an exemplary embodiment the protective layer can include a throughglass via wafer, which has at least one through-contacted contact point.Such through-contacted glass wafers (English: through glass via wafer)are distributed commercially by various companies. Use of such throughglass via wafers can be advantageous because they provide a hermeticallysealed and highly planar packaging material and can also offer majoradvantages in production of the measuring element, since they can, forexample, be processed using the standard methods of MEMS andsemiconductor production.

The protective layer can also have a barrier layer, which is arrangedbetween the through glass via wafer and the ion-sensitive layer. Thebarrier layer can serve to chemically uncouple the through-contact glasswafer from the electrically conductive layer, so that the unwantedchemical reactions between the materials of the electrically conductivelayer and the through glass via wafer can be prevented. In addition, thebarrier layer can permit a defined adhesion for the wafer bondingprocess for bonding to the through glass via wafer.

In another exemplary embodiment, the protective layer is designed andconfigured as a diffusion barrier.

The protective layer and/or the barrier layer can include one or more ofthe following exemplary materials: glass or vitreous materials, inparticular oxidic glass or lithium-phosphorus oxynitride glass and/ormetallic or ceramic materials, which are conductive electronically anddo not form alloys with lithium, as well as mixtures thereof.

Vitreous materials include, for example, silicon dioxide (SiO2), siliconoxides (SiOx), silicon nitride (Si3N4), silicon nitrides (SiNx),spin-on-glass, silicate glass, phosphosilicate glass, fluorophosphateglass, lime-soda glass and/or borosilicate glass. Examples of metallicor ceramic materials that are electrically conductive but do not formalloys with lithium include chromium, nickel, iron, tantalum, zirconium,titanium, hafnium, and their metal nitrite compounds, metal boridecompounds, metal carbide compounds, metal carbonitride compounds as wellas mixed materials, such as titanium, zirconium, nitride in particularor related materials.

Furthermore, the measuring element can include a mechanically stablesubstrate, which serves in particular to stabilize the finishedmeasuring element in order to thereby make it more robust on the whole.

The mechanically stable substrate and the ion-sensitive glass can, forexample, have comparable coefficients of expansion. In this way,stresses in the measuring element due to temperature fluctuations in theenvironment can be minimized or even prevented.

An exemplary mechanically stable substrate preferably can, for example,include one or more of the following materials: metal, steel, ceramic,glass, glass ceramic, polymer compound and fiber composite material.Additional exemplary materials include ceramics, such as zirconiumoxide, nickel-iron alloys and/or glass that is used for the productionof glass shafts of the known ion-sensitive glass electrodes.

In another exemplary embodiment, the mechanically stable substrate is acircuit board such as one having an adjusted coefficient of expansion.This configuration can be advantageous because, for example, contactpoints for tapping the sensor signal may be formed.

If a circuit board, such as one having an adjusted expansion coefficientin combination with a through-contact glass wafer is used as theprotective layer, then the circuit board can be bonded to the glasswafer by means for adhering such as an ACA adhesive (ACA:anisotropically conductive adhesive). The ACA adhesive is supplied, forexample, as ACF tapes (ACF: anisotropically conductive film).

In another exemplary embodiment, the measuring element can also includea temperature sensor, which is embedded in the circuit board, forexample, in the form of a meandering gold/platinum path, or it may beapplied to the surface of the circuit board as an SMT component (SMT:surface mount technology), for example.

In another exemplary embodiment, the measuring element includes apassivation layer, which covers the protective layer, omitting thecontact region, with the passivation layer being arranged between theprotective layer and the stable substrate.

A measuring element as disclosed herein can also have an electriccontact for tapping the measurement signal, the contact being arrangedon or formed in the outer surface of the mechanically stable substrate.

The present disclosure also relates to an ion-sensitive solid contactelectrode for measuring an ion activity in a measurement medium using ameasuring element as disclosed herein.

In addition, the present disclosure relates to an electrochemical sensorfor measuring ion activity in a measuring element and having a referenceelectrode.

FIG. 1 shows a highly schematic view of an exemplary electrochemicalsensor 10 having an ion-sensitive solid-contact electrode 1, having anexemplary measuring element 2 according to the disclosure and having areference electrode 8. The ion-sensitive solid contact electrode 1 caninclude an electrode head 3, which in turn includes at least a portionof the electronic measurement system as means by which the electrode isconnected to a higher-level display and/or control unit 9, for example,a transmitter. This connection may be either by wire or wireless, forexample, inductive. The electrode head 3 is connected to an electrodebody 4, in which the measuring element 2 is arranged so that at least asensitive region is in direct contact with a measurement medium 5. Onthe surface facing away from the measurement medium 5, the measuringelement 2 has a contact 6, for example, a print. A line 7, over which areceived measurement signal can be sent to the electrode head 3, issituated between this contact 6 and the electrode head 3.

FIG. 2 also shows a highly schematic view of an ion-sensitive layerstructure 210 in a sectional view, representing an electrochemicalsolid-state system as part of a measuring element according to thepresent disclosure. This layer structure can include an ion-sensitiveglass layer 211, an electrically conductive layer 212 and a solid-stateelectrolyte layer 213 arranged between the glass layer 211 and theelectrically conductive layer 212.

In the case of a pH electrode, the glass layer 211 can include a knownpH-sensitive glass composition such as that also used as a glassmembrane for known pH glass sensors. The glass layer 211 is, forexample, provided as a glass wafer and has an exemplary thickness ofbetween approx. 0.05 mm and approx. 1 mm. In another exemplaryembodiment the glass layer 211 can be applied to the solid-stateelectrolyte layer 213 by sputtering or by any other known thin layermethod. During a measurement, at least one sensitive region of a surfaceof the glass layer 211 is in direct contact with the measurement medium.

A solid-state electrolyte layer 213 is then applied to the surface ofthe glass layer 211 facing away from the measurement medium. Suitablematerials for the solid-state electrolyte layer 213 includelithium-ion-conducting solid-state compounds. Such solid-state compoundsinclude, for example, lithium borates, such as B2O3-Li2O, lithium boratesulfates (LiBSO), such as B2O3-Li2O, lithium borate-sulfates (LiBSO),such as B2O3-0.8Li2O-0.8Li2SO4, lithium borophosphates (LiOP), such as½×Li2O—BPO4, Li—O-2O3-P2O5 in general, lithium aluminates, such asLi5AlO4, lithium borosilicates, such as Li2O—B2O3-SiO2, lithium galliumoxides, such as Li5GaO4, lithium germanates, such as Li(4-3x)AlxGeO4,lithium nitrides, lithium phosphates, such as Li(1+x)Ti2SixP(3-x)O12 orLi(1+x)MxTi(2-x)(PO4)3, where M=Al, Ga, Fe, Sc, In, Lu, Y, La,lithium-phosphorus oxynitrides, such as Li3PO4-xNx, lithium silicatealuminates, such as LiAlSi2O6, LiAlSiO4, Li9SiAlO8, lithium silicates,such as Li2SiO4, Li4SiO4, Li2SiO3, Li2Si2O5, lithium silicon phosphates,such as Li3.6Si0.6P0.4O4, lithium-silicon-phosphorus oxynitrides, suchas Li4SiO4-x-Li3P4-yNx+y, lithium thiogermanates, Li2GeS3, Li4GeS4,Li6GeS5, lithium titanates, such as Li2Zr(1-x)TixO3, lithium vanadates,lithium compounds, such as Li2S-SiS2-Li3PO4 or Li7P3S11, lithium-zincoxides, such as Li6ZnO4, nitrated lithium borates (Li—B—O—N), such asLi3.09BO2.53N0.52, sulfides, such as Li2S—GeS2, andthio-lithium-germanium compounds, such as Li10GeP2S12,Li3.25Ge0.25P0.75S4 or mixtures thereof.

In an exemplary embodiment, the solid-state electrolyte layer 213 caninclude an alkali metal solid-state electrolyte compound, such as alithium phosphorus oxide nitride compound (LiPON), which is applied bysputtering, for example. Additional methods suitable for applying thesolid-state electrolyte layer 213 include various thin layer methodsincluding among others: pulsed laser deposition, magnetron sputtering,reactive magnetron sputtering, CVD, vapor deposition, reactive vapordeposition, sol-gel methods and plasma-assisted coating methods, such asplasma assisted CVD or vacuum plasma spraying. The solid-stateelectrolyte layer 213 can have an exemplary layer thickness betweenapprox. 50 nm and approx. 5000 nm, for example a layer thickness ofapprox. 100 nm to approx. 1000 nm.

Next, a single-phase electrically conductive layer 212 is applied to thesolid-state electrolyte layer 213 containing pure lithium or an alloycontaining lithium-(0). An exemplary layer thickness of the electricallyconductive layer 212 is between approx. 10 nm and approx. 10 μm. Thesingle-phase, electrically conductive layer 212 is for examplepreferably a solid solution, a solid-state compound or pure substanceand has a high lithium concentration or activity. Examples of suitablelithium-(0)-alloys include single-phase lithium-magnesium alloys orlithium-copper alloys. Because of their higher lithium concentration,lithium-rich single-phase lithium-magnesium alloys with a cubicbody-centered crystal structure are suitable as the electricallyconductive layer. The electrically conductive layer is also, forexample, homogenous.

A measurement principle of an electrochemical electrode with a measuringelement according to the present disclosure is based on an ionicreaction of the measurement medium with the ion-sensitive glass layer211. The solid-state electrolyte layer 213 is also ion conductive andalso allows a reversible redox reaction to take place between ionic andmetallic lithium (Li(0)) at the interface between the solid-stateelectrolyte layer 213 and the electrically conductive layer 212, so thata measurement signal can be picked up by a suitable electrical contactat the electrically conductive layer 212.

The exemplary layer structure 210 shown in FIG. 2 thus constitutes anactual electrochemical solid-state measurement system, which has beenfound in particular to be hysteresis-free and fully reversibleelectrochemically in application. Furthermore, this system has a highexchange current density at the interface between the electricallyconductive layer 212 and the solid-state electrolyte layer 213 and isthermodynamically stable, at least at temperatures between, for example,approx. −40° C. and more than +150° C., which corresponds to a typicaltemperature range for use of pH sensors.

FIG. 3 shows schematically an exemplary measuring element 300 accordingto the present disclosure with an ion-sensitive layer structure 210according to FIG. 2 in a sectional view. The measuring element 300 caninclude an ion-sensitive glass layer 211, which is a thin glass wafer ofan ion-sensitive glass, for example, such as a pH-sensitive glass. Thesolid-state electrolyte layer 213 and an electrically conductive layer212 are applied to this glass layer 211 by one of the aforementionedmethods. Depending on the embodiment of the electrically conductivelayer 212, should be protected from environmental influences. Metalliclithium or pure lithium, for example, is highly reactive and is oxidizedeven by atmospheric oxygen or humidity, which can result in destructionof the measuring element. The measuring element 300 shown here thereforehas a diffusion barrier as a protective layer 314, covering the entiresurface of the electrically conductive layer 212. The diffusion barrier314 may have one or more of the following exemplary materials orcompounds: glass, vitreous materials, such as silicon dioxide (SiO2),silicon oxides (SiOx), silicon nitride (Si3N4), silicon nitrides (SiNx),spin-on glass, silicate glass, phosphosilicate glass, fluorophosphateglass, lime-soda glass and/or borosilicate glass or oxidic glasses withsuitable properties. Also possible are LiPON and related materials,metallic as well as ceramic layers of electronically conductivematerials that do not form an alloy with lithium, such as, for example,chromium, nickel, iron, tantalum, zirconium, titanium, hafnium as wellas their metal nitrides, metal borides, metal carbides, metalcarbonitrides as well as mixtures thereof, such as titanium zirconiumnitride or related materials.

In an exemplary embodiment, the diffusion barrier 314 is applied, forexample, via a reactive magnetron sputter process. In addition, oralternately, the diffusion barrier 314 can be applied by any of thefollowing exemplary methods: lamination methods, anhydrous sol-gelcoating, spin coating, plasma spraying, vacuum plasma spraying,sputtering, CVD or PVD methods, such as vapor deposition, reactive vapordeposition, ion beam-assisted deposition, ion plating or pulsed laserdeposition.

As FIG. 3 shows, the exemplary solid-state electrolyte layer 213, theelectrically conductive layer 212 and the diffusion barrier 314 do notcover the entire surface of the glass layer 211, and furthermore, theyare covered with a passivation layer 316 except for a contact region315. The passivation layer 316 serves as additional protection for theelectrically conductive layer 212 and can include, for example, one ormore of the following compounds: metal, in particular steel, ceramic,glass, glass ceramic, polymer compound, fiber composite material orcombinations thereof.

The layer structure created to this extent can be bonded to amechanically stable substrate 318 by means for adhering, such as aconductive adhesive 317. The mechanically stable substrate 318 caninclude, for example, one or more of the following compounds: metal, inparticular steel, ceramic, in particular zirconium dioxide ornickel-iron alloys, glass, glass ceramics, polymer compounds, fibercomposite materials or circuit boards, wherein the mechanically stablesubstrate 318 preferably has an expansion coefficient comparable to thatof the ion-sensitive glass layer 211. Exemplary expansion coefficientsof substrate 318 and of glass layer 211 preferably do not differ fromone another by more than for example approx. 10% in order to ensureenhanced thermal stability of the measuring element.

The measurement signal is tapped via a contact 306, which is formed onor attached to the surface of the substrate 318 (see also FIG. 1).

In addition, the measuring element can include a temperature sensor 32,which is attached to the substrate 318 as an SMT component, asillustrated here.

FIG. 4 shows another exemplary embodiment of a measuring element 400according to the present disclosure having a layer structure 210. Thelayer structure 210 includes an ion-sensitive glass layer 211, which inthis embodiment includes (e.g., consists of) a pH-sensitive glass and isfor example approx. 200-μm thick. The glass layer 211 is covered with asolid-state electrolyte layer 213 to, for example, approx. 1000 μm,except for a peripheral border area 440 of for example approx. 200 μm,and this layer is then covered completely with an electricallyconductive layer 212. The solid-state electrolyte layer 213 here caninclude a lithium-phosphorus oxide nitride compound in a thickness of,for example, approx. 1000 nm and the electrically conductive layer 212can include pure metallic lithium in a thickness of for example approx.1000 nm.

In an exemplary embodiment, the electrically conductive layer 212 iscovered completely by a barrier layer as the protective layer 430, whichin this embodiment is applied to a through glass via wafer 432. Thisbarrier layer 430 can also serve as a contact layer between theelectrically conductive layer 212 and the through glass via wafer 432,which here includes a plurality of through-contacted contact points 436.

Through glass via wafers are available commercially from variousproviders. The glass wafer 432 is connected to the layer structure 210via the barrier layer 430 and a bonding metallization 431, whichcontacts the peripheral bordering edge 440 of the ion-sensitive glasslayer 211. The barrier layer 430 and the bonding metallization 431, forexample, includes one of the following metals or metal compoundscontaining these metals: titanium, chromium, nickel, gold, platinum.

Furthermore, a circuit board, which is also referred to as a PCB(printed circuit board), is applied as a mechanically stable substrate418 by means for adhering, such as an adhesive layer 433, namely ACAadhesive here (ACA: anisotropically conductive adhesive). The circuitboard is, for example, a CTE-matched PCB (CTE: coefficient of thermalexpansion), in which the coefficient of thermal expansion of the circuitboard is adapted to that of the ion-sensitive glass 211. A measurementsignal occurring on the basis of the interaction of a measurement mediumwith the ion-sensitive glass membrane 211 can be picked up by a contactregion (not shown here) (see FIGS. 1 and 3).

FIG. 5 shows an exemplary measuring element 400 according to FIG. 4 inthe form of two modules 400A and 400B, wherein module 400A includesessentially the layer structure 210, and the module 400B can include themechanically stable substrate 418 with the barrier layer 430, theadhesive layer 433, the through glass via wafer 432 and the bondingmetallization 431. As shown here in a highly schematic form, the modules400A and 400B are created essentially independently of one another andonly then are bonded together. This can be advantageous, so thatproduction rejects in particular can be greatly reduced by producingmodule 400A, which is more sensitive to environmental influences,independently of module 400B.

FIG. 6 shows another exemplary measuring element 500 according to thepresent disclosure with an ion-sensitive layer structure 210, which isconstructed like the measuring unit 300 (see FIG. 3). The electricallyconductive layer 212 is covered with a diffusion barrier as theprotective layer 514, which is also covered by an insulator layer 516,omitting a contact region 515, like the peripheral border 540 of theglass layer 211. In this case, a circuit board with a thermal expansioncoefficient CTE of for example approx. 10 ppm/K as a mechanically stablesubstrate 518 is bonded to the insulator layer 516 by means foradhering, such as an ACF tape (ACF: anisotropically conductive film).Furthermore, a contact 506 for picking up the measurement signals can beformed in the mechanically stable substrate 518. The measuring element500 also can include a temperature sensor 520, which is formed in thesubstrate 518.

FIG. 7 shows a comparison of the raw data of known measurement signals(raw sensor signal) of a known glass sensor and an ion-sensitive solidcontact electrode according to the present disclosure at 25° C. anddifferent pH levels. The measurement signals were determined withrespect to an Ag/AgCl reference electrode, wherein FIG. 7A shows thetheoretical values for a known glass sensor, and FIG. 7B shows thevalues measured with an ion-sensitive solid-contact electrode accordingto the present disclosure.

FIG. 7B shows that the measured potential of an ion-sensitivesolid-contract electrode according to the present disclosure is in alinear ratio with respect to the pH level with a Nernst slope. Incomparison with the known glass sensor (see FIG. 7A), the measurementsignals of the ion-sensitive solid-contact electrode (see FIG. 7B) havea somewhat lower slope of 59.1 mV/pH at 25° C. This can, for example, beattributed to the experimental circumstances. Furthermore, the offsetpotential of the ion-sensitive solid-contact electrode differs from thatof a glass sensor and is for example approx. −2980 mV in comparison withapprox. 0 mV with a known glass sensor. This deviation can be attributedmainly to the asymmetry of the ion-sensitive solid contact electrode.

It will be appreciated by those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restricted. The scope of the invention is indicated by theappended claims rather than the foregoing description and all changesthat come within the meaning and range and equivalence thereof areintended to be embraced therein.

LIST OF REFERENCE NUMERALS

-   -   1 electrode    -   2 measuring element    -   3 electrode head    -   4 electrode body    -   5 measurement medium    -   6, 306, 506 contact    -   7 line    -   8 reference electrode    -   9 display/control unit    -   10 electrochemical sensor    -   210 layer structure    -   211 ion-sensitive layer    -   212 electrically conductive layer    -   213 solid-state electrolyte layer    -   300, 400 measuring element    -   314, 514 protective layer/barrier layer    -   315, 515 contact region    -   316, 516 passivation layer    -   317 adhesive layer    -   318, 418, 518 substrate    -   320, 520 temperature sensor    -   430 barrier layer/protective layer    -   431 bonding metallization    -   432 glass wafer    -   433 adhesive layer    -   435 contact region    -   436 contact point    -   440, 540 boundary area

What is claimed is:
 1. A measuring element for an ion-sensitivesolid-contact electrode for measuring ion activity in a measurementmedium, the measuring element comprising: an ion-sensitive layer,arranged to contact a measurement medium when in operation, andconductive for lithium ions; and a single-phase electrically conductivelayer which includes metallic lithium or a lithium-(0)-alloy, whereinthe measuring element is a solid-state measuring element which includesa solid-state electrolyte layer arranged between the ion-sensitive layerand the electrically conductive layer.
 2. The measuring elementaccording to claim 1, wherein the ion-sensitive layer comprises: anion-sensitive glass, which is conductive for lithium ions.
 3. Themeasuring element according to claim 1, wherein the lithium-(0)-alloy ofthe single-phase electrically conductive layer is a single-phaselithium-magnesium alloy, a lithium-copper alloy or a mixture thereof. 4.The measuring element according to claim 3, wherein thelithium-(0)-alloy is a lithium-rich single-phase lithium-magnesium alloywith a cubic body-centered crystal structure.
 5. The measuring elementaccording to claim 1, wherein the solid-state electrolyte layercomprises one of the following: Li-ion-conducting solid-state compoundsor mixtures thereof; any lithium borates, including B2O3-Li2O; anylithium borate-sulfates (LiBSO), including B2O3-0.8Li2O-0.8Li2SO4; anylithium borophosphates (LiOP), including ½×Li2O—BPO4, Li—O-2O3-P2O5; anylithium aluminates, including Li5AlO4; any lithium borosilicates,including Li2O—B2O3-SiO2; any lithium gallium oxides, including Li5GaO4;any lithium germanates, including Li(4-3x)AlxGeO4; any lithium nitrides;any lithium phosphates, including Li(1+x)Ti2SixP(3-x)O12 orLi(1+x)MxTi(2-x)(PO4)3 where M=Al, Ga, Fe, Sc, In, Lu, Y, La; anylithium-phosphorus oxynitrides, including Li3PO4-xNx anylithium-silicate aluminates, including LiAlSi2O6, LiAlSiO4, Li9SiAlO8;any lithium silicates, including Li2SiO4, Li4SiO4, Li2SiO3, Li2Si2O5;any lithium-silicon phosphates, including Li3.6Si0.6P0.4O4; anylithium-silicon-phosphorus oxynitrides, including Li4SiO4-x-Li3P4-yNx+y;any lithium thiogermanates; any Li2GeS3, Li4GeS4, Li6GeS5; any lithiumtitanates, including Li2Zr(1-x)TixO3; any lithium vanadates; any lithiumcompounds, including Li2S-SiS2-Li3PO4 or Li7P3S11; any lithium-zincoxides, including Li6ZnO4; any nitrated lithium borates (Li—B—O—N),including Li3.09BO2.53N0.52; any sulfides, including Li2S—GeS2; or anythio-lithium-germanium compounds, including Li10GeP2S12,Li3.25Ge0.25P0.75S4.
 6. The measuring element according to claim 5,wherein the solid-state electrolyte layer comprises: anylithium-phosphorus oxynitride compound, including Li3PO4-xNx.
 7. Themeasuring element according to claim 1, wherein the measuring elementcomprises: a protective layer applied to the electrically conductivelayer as protection against environmental influences.
 8. The measuringelement according to claim 7, wherein the protective layer comprises: athrough glass via wafer having at least one through contact point via.9. The measuring element according to claim 8, wherein the protectivelayer comprises: a barrier layer arranged between the through glass viawafer and the electrically conductive layer.
 10. The measuring elementaccording to claim 7, wherein the protective layer is a diffusionbarrier.
 11. The measuring element according to claim 1, wherein theprotective layer and/or the barrier layer comprises at least one of thefollowing materials: glasses, glassy materials, oxidic glass,lithium-phosphorous oxynitride compounds and/or metallic or ceramicmaterials, which are electronically conductive and do not form alloyswith lithium, as well as mixtures thereof.
 12. The measuring elementaccording to claim 1, wherein the measuring element comprises: amechanically stable substrate, wherein the mechanically stable substrateand the ion-sensitive layer have similar expansion coefficients.
 13. Themeasuring element according to claim 12, wherein the mechanically stablesubstrate comprises: one or more of the following materials: metal,steel, ceramic, glass, glass ceramics, polymer compound, or fibercomposite material.
 14. The measuring element according to claim 12,wherein the mechanically stable substrate is a circuit board.
 15. Themeasuring element according to claim 1, comprising: a temperaturesensor.
 16. The measuring element according to claim 1, comprising: apassivation layer which covers the protective layer, omitting a contactregion, with the passivation layer being arranged between the protectivelayer and the mechanically stable substrate.
 17. The measuring elementaccording to claim 12, comprising: an electric contact for picking up ameasurement signal, said contact being arranged on or formed in anoutside surface of the mechanically stable substrate.
 18. Anion-sensitive solid-contact electrode for measuring ion activity in ameasurement medium having a measuring element to claim
 1. 19. Anelectrochemical sensor for measuring ion activity in a measurementmedium comprising: an ion-sensitive solid-contact electrode according toclaim 18; and a reference electrode.